1. Technical Field
The present invention relates to a highly sensitive magnetostrictive device used in a non-contact magnetostrictive torque sensor and a method of manufacturing the same.
2. Related Art
A magnetostrictive torque sensor 10 has been known which uses a magnetostrictive device 1 formed with a magnetic anisotropic portion 15, as shown in FIG. 1 later. In this magnetostrictive torque sensor 10, the permeability of the magnetic anisotropic portion 15 changes according to the magnitude of a torque applied to the magnetostrictive device 1. A change in the permeability is detected as by a detection coil 23 to determine the magnitude of the applied torque.
It is desired that the magnetostrictive device 1 of such a magnetostrictive torque sensor 10 be excellent and stable in its magnetic characteristic.
The official gazette of JP-B-7-10011 discloses a magnetostrictive device manufacturing method which forms a compression-hardened layer in the entire magnetic anisotropic portion (magnetostrictive portion) of the magnetostrictive device by shot-peening the magnetic anisotropic portion.
With this conventional technique, it is said to be possible to obtain a magnetostrictive device with a small hysteresis and a high detection sensitivity.
The magnetostrictive device produced by the conventional manufacturing method, however, has the following problem.
The shot peening of the whole magnetic anisotropic portion may change the groove shape of the magnetic anisotropic portion in a direction that degrades the sensitivity.
That is, the shape formed by the shot peening is difficult to control. The shot peening deforms projections in a direction that flattens them and edges in a direction that rounds them. Hence, when the magnetic anisotropic portion having troughs and ridges arranged alternately is subjected to the shot peening, the ridges may get lower in height than expected or rounded more than necessary.
The magnetostrictive device passes a magnetic flux through the ridges in the magnetic anisotropic portion to measure its distortion. When the excitation frequency in the torque sensor is in a high frequency range of, for example, about 50 kHz, the magnetic flux concentrates on the vertices of the ridges. Therefore, the larger the area of the top of the ridge, the larger the area through which the magnetic flux passes and the greater the contribution to the improvement of the sensor sensitivity.
When the shot peening is performed, the crest is rounded as described above, making it difficult to increase the flat area at the crest. For this reason, in the high frequency region, the shot peening reduces the area through which the flux can pass and lowers the sensor sensitivity.
Further, a change in the height of the ridge caused by the shot peening changes the air gap between the ridge and the coil arranged opposed to the ridge. This in turn leads to a reduction in the sensor sensitivity.
The torque sensor using the magnetostrictive device obtained by the shot peening has a problem of poor stability against excess load torques and low sensor sensitivity when the excitation frequency is set in a low frequency region (for example, 20-30 kHz). This is considered due to the fact that in a low frequency region the flux penetrates relatively deep into the ridge, whereas the shot peening can only strengthen the surface of the ridge but not a relatively deep part into which the flux penetrates.
The magnetic anisotropic portion that has undergone the conventional shot peening has a problem of low sensor sensitivity not only when the excitation frequency is high but when it is low.
The present invention has been accomplished with a view to overcoming the conventional drawback and provides a magnetostrictive device whose sensor sensitivity is high whether the excitation frequency is in the high frequency region or in the low frequency region and which can withstand an excess load torque, and also provides a method of easily manufacturing the magnetostrictive device.
According to an aspect of the invention, there is provided a method of manufacturing a magnetostrictive device for a torque sensor having a magnetic anisotropic portion formed on a surface of a shaft body thereof, wherein the magnetic anisotropic portion is formed by forming spiral grooves in the surface of the shaft body to form a plurality of troughs and ridges alternately and by directly applying a compressive stress only to the ridges to cause deformations accompanied by plastic flow and provide an almost planar plateau portion at a crest portion of each of the ridges.
What should be noted most in this invention in forming the magnetic anisotropic portion is to, after forming the plurality of troughs and ridges alternately, directly apply a compressive stress to only the ridges to form the plateau portion.
The forming of the grooves in the shaft body can be done by a variety of methods, including a plastic forming process such as form rolling and a cutting process.
The troughs and ridges made by the forming of the grooves may be triangular, trapezoidal or of other shapes. The grooves are formed as spiral grooves at two locations in the shaft body in a conventionally known pattern in which the two sets of spiral grooves are inclined at the same angles (almost 45xc2x0) in different directions.
Next, only the ridges are applied directly with a compressive stress. This compressive stress is not applied to the troughs. Such a compressive stress can be applied easily by using a forming process described later, such as rolling and swaging, that uses a tool.
The ridges are applied with the compressive stress to cause deformations accompanied by plastic flow. This forming process is performed to form almost planar plateau portions. That is, if the ridges before being applied with the compressive stress are triangular in shape, the forming process flattens the crests of the ridges to form them into a trapezoidal shape. When the ridges before being applied with the compressive stress are trapezoidal in shape, the forming process further expands the plateau portions of the crests to form the ridges into a trapezoidal shape. When the ridges before being applied with the compressive stress are of sine waveform in shape, the forming process flattens the crests and nearby portions to form them into an almost trapezoidal shape.
The material of the shaft body may use any material applicable to the torque sensor""s magnetostrictive device. In more specific terms, the material includes Fexe2x80x94Cr alloys and Fexe2x80x94Nixe2x80x94Mo alloys.
Next, the workings of the invention will be described.
In this invention, only the ridges are applied with the compressive stress to cause deformations accompanied by plastic flow and thereby provide the plateau portions. Unlike the conventional shot peening which rounds the ridges, the compressive stress used In this invention is applied to Increase the area of the plateau portion at the crest of the ridge. The increase in the area of the crest portion results in an increase in the flux penetrating area when the excitation frequency is high. Hence, the torque sensor""s magnetostrictive device thus obtained has a very high sensor sensitivity when the excitation frequency is high.
The plateau portion is formed by the deformation accompanied by the plastic flow. Therefore, underneath the plateau portion, the residual compressive stress can be applied to portions at relatively large depths from the surface.
That is, when the conventional shot peening is performed, the residual compressive stress remains at the surface layer but sharply decreases as the depth increases.
In this invention, by contrast, the deformations accompanied by plastic flow can leave the residual compressive stress even in relatively deep portions below the plateau portion.
Therefore, when the magnetostrictive device is applied with an excess load torque, the influence of the applied torque can be suppressed in portions down to a relatively large depth below the ridge. This in turn can maintain a high sensor sensitivity even when the excitation frequency is relatively low and the magnetic flux penetrates relatively deep into the ridges.
Hence, it is possible to provide a magnetostrictive device for a torque sensor which has a high sensor sensitivity whether the excitation frequency is in the high frequency region or in the low frequency region and can withstand an excess load torque satisfactorily and to provide a method of easily manufacturing such a magnetostrictive device.
In the above manufacturing method, a heat treatment process may be added either before or after the groove formation process, as required. In that case, the microstructure of the shaft body has an improved stability against the excess load torque.
According to another aspect of the invention, it is preferred that the application of the compressive stress to the ridges be performed by using a rolling process or a swaging process. In that case, the degree of forming and the shape of the ridge can be controlled with precision, which in turn facilitates the forming of the plateau portion.
According to still another aspect of the invention, it is preferred that the forming of the spiral grooves and the application of the compressive stress be performed continuously by using one tool. This enables a significant reduction in the number of processes and therefore a reduced manufacturing cost. More specifically, it may be possible to use a tool that integrally combines a form-rolling tool and a rolling tool.
According to a further aspect of the invention, it is preferred that the compressive stress be applied to the ridges so that the relation 0.2 less than (H0-H1)/H0 less than 0.48 holds where H0 is a height of the ridges before being applied with the compressive stress and H1 is a height of the ridges after being applied with the compressive stress. In that case, the plateau portions of the ridges can be hardened reliably and the stability against excess load torque improved.
According to a further aspect of the invention, it is preferred that the shaft body be applied with an excess load torque one or more times after the magnetic anisotropic portion is formed. That is, the magnetostrictive device should preferably be preloaded with an excess load torque one or more times before it is put to actual use. This can further improve the stability against the excess load torque.
The excess load torque is a torque which exceeds the measuring range (normal use range) of the magnetostrictive device.
Next, according to a further aspect of the invention, there is provided a magnetostrictive device for a torque sensor having a magnetic anisotropic portion formed on a surface of a shaft body thereof, wherein the magnetic anisotropic portion has a plurality of troughs and ridges arranged alternately, the troughs and ridges being formed by forming spiral grooves in the surface of the shaft body, the ridges each having an almost planar plateau portion, the plateau portion being formed by directly applying a compressive stress only to the ridges and deformations accompanied by plastic flow.
What should be noted most in this invention is that the ridges are formed with the plateau portions by the forming method described above.
The presence of the plateau portions, as described above, can increase the flux penetrating area when the excitation frequency is high. Thus, the torque sensor""s magnetostrictive device thus obtained has a very high sensor sensitivity.
Because the plateau portions are hardened by the compression deformation accompanied with the plastic flow, the magnetostrictive device is not easily deformed permanently when applied with an excess load torque. This in turn improves stability against the excel load torque.
With the present invention, therefore, it is possible to provide a magnetostrictive device for a torque sensor which has a high sensor sensitivity even when the excitation frequency is in the high frequency range.
According to a further aspect of the invention, it is preferred that the residual compressive stress in the plateau portion be 30 kgf/mm2 or higher at the surface and 20 kgf/mm2 or higher at a depth of 0.3 mm from the surface.
When the residual compressive stress is less than 30 kgf/mm2 at the surface of the plateau portion, there is problem of degraded stability against the excess load torque. On the other hand, because the residual compressive stress on the surface does not degrade the magnetic characteristic significantly, the upper limit of the residual compressive stress should preferably be around 80% of the tensile strength of the material.